Critical properties of a dense polycrystalline (Ba, K)Fe2As2 superconductor prepared by a combined process of melting and deformation

We report the synthesis of a polycrystalline bulk (Ba, K)Fe2As2 superconductor by a combined process of melting and deformation. The sample has fewer impurity phases and a higher density compared to samples prepared by a conventional sintering process. Magneto-resistance and magnetization measurements were carried out on the sample in order to evaluate superconducting critical properties. The temperature dependence of the upper critical field, - dHc2(T)/dT, is as large as ∼ 9 T K-1, which corresponds to the value of a single crystal in a magnetic field parallel to the a-b plane. The temperature dependence of irreversibility fields, - dHirr(T)/dT, also has a steep slope at about 10 K below Hc2(T), suggesting that (Ba, K)Fe2As2 superconductors have a good potential for magnetic applications at medium temperatures, like MgB2. However, the magnetization hysteresis loop shows a very small dependence on sample size and the global Jc estimated is as low as ∼ 2 x 103 A cm-2 at 5 K in applied magnetic fields, while the local Jc estimated from the powdered sample is about two orders of magnitude higher. Those results indicate that the current path is still largely limited in the bulk sample despite its high density, and a weak link at the grain boundary is the most plausible origin of the current limitation in the bulk sample.